Field of the Invention
The present invention relates to an electrode assembly for an electrical impedance tomography (EIT) scanning device, a belt-like device comprising a plurality of said electrode assemblies and a method of measuring an electrical impedance tomography image using a plurality of electrode assemblies arranged in a spaced apart relationship.
Prior Art
Electrical impedance tomography (EIT) is a non-invasive imaging technique used to investigate and measure the composition and function of opaque bodies, including living species. Recently, EIT has been successfully applied in patients. For intensive care doctors, pulmonologists, and physiotherapists, electrical impedance tomography (EIT) is an imaging method that provides real-time information about regional lung ventilation and perfusion (flow of blood). In contrast to conventional methods, EIT does not require the patient to breathe through a sensor, does not apply ionizing x-rays, and can be used for extended periods, say 24 hours or even longer. Therefore, EIT can be used continuously and is therefore suited for monitoring treatment effects in real time and over time. EIT was first used to monitor respiratory function in 1983 and remains the only bedside method that allows continuous, non-invasive measurements of regional changes in lung volumes, blood flow, and cardiac activity. More details to this technique can be found in the article “Electrical impedance tomography” by Costa E L, Lima R G, Amato M B. in Curr Opin Crit Care. 2009 February; 15(1):18-24, which is incorporated herein by reference.
In EIT, as disclosed by U.S. Pat. No. 5,626,146, a plurality of electrodes, typically from 8 to 32, are arranged around the chest of a subject. A control unit ensures that an electrical signal, for example a current is applied to one or several pairs of electrodes on the skin to establish an electrical field which in turn is measured by the other electrodes. The electrodes used to apply current are called “current injecting electrodes” although one of them might serve as reference potential, for example ground. Typically, 3 to 10 mA are injected at 50 to 200 kHz. With the remaining electrodes, the resulting voltages are measured and subsequently used to estimate the distribution of electric impedance within the thorax. Specific algorithms were developed to convert the set of voltages into images. In order to overcome the ill-posed nature of impedance estimation, most EIT imaging algorithms make use of additional assumptions, known as a priori information and regularization. A priori information can be, for example, geometrical information about the subject chest. A typical regularization assumption is that intra-thoracic impedance distribution shall have no abrupt changes. Another assumption is that all electrodes are properly connected to the skin of the patient. The resulting images provide a reasonable estimation of the true impedance distribution within the thorax.
To be useful for the user, for example the clinician, the calculated impedance-distribution image is converted into an image that shows presence of air, absence of air, or changes of air content, changes of blood content, and/or muscular excursion into body spaces. Instead of air, any breathable gas mixture can be used in EIT. Plotted rapidly in sequence, several times per second and like a movie, these images create a visual representation of gas and blood flow in and out of each lung region and allow the doctor to evaluate lung function and/or cardiac function in real-time. Current main obstacles to wide spread use of EIT are poor reliability and high cost.
Electrodes can disconnect easily and electrical contact with the skin may vary by orders of magnitude, from as much as 500 Ohms to 10,000 Ohms. Poorly connected electrodes are unable to inject the necessary pre-defined current due to voltage supply limits. Consequently, the resulting images often contain artefacts due to in-homogeneous current injection. Such artefacts can be mistaken as physiologic signals and potentially lead to wrong diagnosis and inappropriate therapy. An example of artefacts due to bad electrode contact is given in FIG. 6b. Even though the user may know that the electrodes make bad contact, he/she might not know what appropriate corrective action might be done.
Rigaud et al. describe in the article “Experimental acquisition system for impedance tomography with active electrode approach” (in Med Biol Eng&Comput, November 1993, 31:593-599) an experimental EIT measuring system comprising an acquisition system including a plurality of so-called “active electrodes” and a control unit including a reference voltage source and an acquisition circuit. The active electrodes comprise each a voltage buffer, a set of switches and a switch logic unit, controlled by the acquisition system. Close to the electrode a current source, the voltage buffer and the set of switches are connected. By these means the electrodes become multifunctional.
In Rigaud the switch logic units of the electrodes are connected to data lines in parallel. Thus each electrode needs appropriate addressing, digital storage, and synchronization. As each electrode must have a unique address, a programmable microchip or special hardwired addressing must be used for this purpose. This makes the manufacture of the control circuitry for the electrodes costly.
The solution proposed by Rigaud assumes that all electrodes are always working well. In their paper, there is no provision disclosed to check the proper function of the electrodes. Also, there is no mention of a method to identify electrodes that are not in good contact with the skin. Non properly functioning electrodes or electrodes with a poor skin contact however have a detrimental effect on the results and should therefore be accounted for.
In Rigaud the differential voltage of two electrodes is hard-wired to a hardware demodulator with subsequent low pass filter. This has the advantage that demodulation is quick. The disadvantage is that the hardware implementation of the analog demodulator is costly and less flexible.
The apparatus disclosed by Rigaud contains as many current sources as electrodes. These current sources are switched on and off by a central computer. Theoretically and practically, more than one current source can be active at the same time thereby injecting potentially life threatening levels of current. Rigaud does not disclose any method to prevent the simultaneous injection of currents in their distributed electrode arrangement.
In one implementation of an intrinsically safe EIT for the purpose of monitoring progress during pressure filtration, as disclosed by York et al. in “An Intrinsically Safe Electrical Tomography System”, Industrial Electronics, 2003, ISIE '03, 2003 IEEE International Symposium on 9-11 Jun. 2003, vol. 2, pages 946-951, and/or in “Towards Process Tomography for Monitoring Pressure Filtration”, IEEE Sensors Journal, vol. 5, no. 2, April 2005, a plurality of electrodes, typically between 16 and 24, are arranged around the subject of interest, in this case said subject of interest is a vessel for pressure filtration in a chemical plant for the separation of a liquid from a solid phase. In this apparatus according to York et al four relays are dedicated to control the electrical signals of any given electrode, one to connect the electrode to the positive line of the current source, one to connect to the negative line of the current source, one to connect to the positive line of the voltmeter, and one to connect to the negative line of a voltmeter. Control of these 4 relays is done by arranging them as one single-bit shift register in a daisy chain and controlling the daisy chain with a connected PC. The mentioned slowness of the measurements, i.e. a rate of about 1-2 images per minute, signifies no disadvantage for the purpose of monitoring pressure filtration processes. Although it is stated that the EIT system described by York et al could be readily applied to other purposes besides the monitoring of progress during pressure filtration, a number of problems would arise if said system were applied for medical purposes. Some of the reasons are as follows: (a) the huge distance between the amplifiers for measuring the voltages and the electrodes, typically 50 meters, creates problems with regard to interference and noise; (b) the low image rate is not acceptable, because the changes in a living body occur with a much faster rate (e.g. breathing or heartbeat rate); and (c) the life time of the electromechanical relays used by York is limited, typically 5 million cycles, due to mechanical use of the parts, which is not acceptable for medical measurements.
US application 2005/0059901 A1 discloses a method to improve reliability of measurement in the frequency domain. The method proposes to search for a frequency of the injection current that provides an optimal signal to noise ratio. It does not take into account that an electrode may fail to make contact with the skin. The said disclosed method will therefore not work if electrodes do not make contact with the skin.
It is known that contact impedance can be calculated as the quotient of the measured voltage and current. However, such measurement does not help to decide when an electrode makes sufficient contact to obtain reliable EIT images. Comparison with expected impedance does not help either since the current injecting device may be beyond its range of operation and thus induce measurement artefacts.
It has been suggested to display the failing electrode on a graphical user interface and/or to trigger an alarm sound to make the user aware of the situation. Neither of these measures prevents the calculation of potentially misleading images.
Another disadvantage of current EIT systems is the high cost. EIT systems measure the voltages for each electrode separately to average the noise inherent in the measurements. Such implementation is rather expensive since it requires as many measurement channels as there are electrodes. A much less expensive implementation would be to measure the voltages around the chest one by one, i.e. employ multiplexing. This technique is known in the art and allows the sharing of significant resources such as expensive differential amplifiers and fast analog-to-digital converters. However, there is significant noise in the measurements that can be eliminated by averaging. For this reason it is advantageous to maximize measurement time. For example, if one of the electrodes is failing, the time allocated for such failing electrode is completely wasted. It would be of great advantage to have a method to exclude non-functioning electrodes from the measurement sequence in order to maximize the time for the functioning electrodes.
A device that is able to provide reliable EIT images in presence of non-contacting electrodes has not been described and is thus currently not available. For the reasons described above, there is a clear need for such robust device to avoid misinterpretation of images and inappropriate therapy.
Accordingly, it is an advantage of the present invention to provide a reliable electrode assembly which can be manufactured at low costs. Another advantage is to provide an assembly circuit for an electrode which does not require prior addressing and expensive electronic parts. A still further advantage is to propose a device and a method by which non-functioning electrodes can be excluded from the measurement sequence. Another advantage is to provide a device and a method that create reliable EIT images in presence of erroneous data due to non-functioning or poorly functioning electrodes. Another advantage is to provide a device and a method that create reliable EIT images for the purpose of medical examination, particularly of lung and heart. Another advantage is to increase the versatility of EIT method, in particular for medical examination. Furthermore another advantage is to independently control the patterns of current injection and the patterns of voltage readout.